The developing Evolved Universal Terrestrial Radio Access (E-UTRA) standards, as promulgated the Third Generation Partnership Project (3GPP), contemplate sending downlink control information (DCI) to targeted mobile stations according to varying message formats. As the amount and type of information conveyed in a given one of these downlink messages depends on the message content as particularized for the targeted mobile station, the message format (e.g., size, encoding, and/or sub-frame positioning) can vary from message to message. Example details for possible DCI message formats are given in Section 5.5.3 of 3GPP TS 36.212 V8.2.0 (2008-03).
That technical specification explains that each DCI message transports downlink or uplink scheduling information, or uplink power control commands for one Medium Access Control (MAC) ID. The MAC ID of the targeted mobile station is implicitly encoded in the Cyclic Redundancy Check (CRC) of each message. While this arrangement is advantageous from a signaling efficiency perspective, it imposes significant challenges at the mobile stations.
In particular, the mobile stations listening for DCI messages do not have a priori knowledge regarding the formatting details used to transmit a given DCI message, nor in general terms do they know in advance which mobile station is targeted by given DCI message. As such, an individual mobile station is obligated to blindly decode DCI messages to see whether a given received DCI message is targeted to it. However, because of the large number of format variations that can be used for sending DCI messages, the mobile station is obligated to test a large number of format assumptions before concluding that the DCI message is not targeted to it. That is, the mobile station faces the twofold challenge of not knowing whether the DCI message is targeted to it, and not knowing the formatting particulars of the decoding message. Thus, a decoding failure may arise from using the wrong formatting assumptions for the message, from too many decoding errors in the received codeword, or because the message is not targeted to the mobile station.
One approach to reducing the “search space” of DCI message decoding in the context of Long Term Evolution (LTE) air interface details is based on limiting the number of “control channel elements” (CCEs) from which DCI messages can be formed, and correspondingly limiting the “starting positions” that can be used for the different aggregation levels. Each CCE includes some number of resource elements (REs). In turn, each RE spans one sub-carrier in frequency and one OFDM symbol in time. Each CCE thus represents a basic resource unit for transmitting control information, and DCI messages of different sizes are accommodated by aggregating different numbers of CCEs. Thus, the number of CCEs aggregated to form a given DCI message represents one message formatting variable that is generally not known a priori to the mobile stations receiving the DCI message. Note, too, that for a given DCI format, the code rate of a DCI message is defined by the number of CCEs that are aggregated to form it.
To limit the search hypotheses implicated by varying CCE aggregations, the number of DCI message format assumptions that must be considered by receiving mobile stations can be limited only to defined CCE aggregations, such as aggregations of 1, 2, 4, or 8 CCEs. FIG. 1 illustrates example aggregations for blocks of six CCEs (CCE1 . . . CCE6). For aggregations of one CCE, there are six possible CCE locations/patterns within the control channel region of a sub-frame, three CCE locations/patterns for CCE aggregations of two CCEs, and two locations/patterns for CCE aggregations of four CCEs. Accordingly, a given mobile station can limit its searching within the control channel region of a sub-frame to the patterns/locations possible for this limited set of CCE aggregations. Still, even with limiting the CCE aggregations that can be used, the universe of message format possibilities is quite large. The search burden can be computationally expensive, to the extent that mobile station battery life is compromised, or even to the extent that the mobile station literally does not have the processing speed necessary to cover the search space in the allowed time.